Method for producing honeycomb sandwich panel

ABSTRACT

A honeycomb sandwich panel is produced by (a) mounting a honeycomb core on a lower prepreg laminate skin placed on a forming die, (b) superimposing thereon an upper prepreg laminate skin to cover the honeycomb core, (c) mounting flat plate-shaped elastic jigs on a portion of the resulting assembly where the lower prepreg laminate skin and the upper prepreg laminate skin are overlapping each other, (d) superimposing flat plate-shaped press jigs on the elastic jigs, and (e) heating the assembly under pressure for adhesion forming.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for producing ahoneycomb sandwich panel suited for the production of a panel structurehaving a curved surface; particularly to a method for producing ahoneycomb sandwich panel while preventing a honeycomb core fromcrushing, thereby simplifying the production process and thus reducingthe cost and weight of the product.

BACKGROUND OF THE INVENTION

[0002] Honeycomb sandwich panels constituted by a pair of prepreg skinsand a honeycomb core interposed between the skins are light in weightand superior in strength and rigidity; therefore, they are used asstructural members of aircraft, automobiles, etc. These honeycombsandwich panels are generally produced by first cutting a honeycombmaterial into a desired shape to form a honeycomb core, covering bothsides of the honeycomb core with prepreg laminate skins, mounting theresulting assembly on a forming die, if necessary, with an adhesivebetween the honeycomb core and the prepreg laminate, and heating theassembly under pressure for adhesion forming. In this production method,however, the edge of the honeycomb core is unable to withstand theforming pressure applied, resulting in frequent deformation (crush) ofthe edge. Therefore, it is desired to develop a method for producing ahoneycomb sandwich panel while preventing the crush of core edge.

[0003] It is known that the crush of honeycomb core is caused byslipping of the prepregs in the skins. Japanese Patent Publication No59-42621, for example, discloses a method for preventing the aboveslipping by causing projections (slipping-preventing means) of a formingdie to pierce into a portion of the honeycomb sandwich panel where alower prepreg laminate and an upper prepreg laminate are overlappingeach other, inserting the projections into a press plate made of anelastic material, thereby fixing the prepreg laminates on and above theforming die. In this method using a forming die having projections,however, the projections are dangerous to an operator; the formingoperation is troublesome because periodic cleaning and repair forprojections are necessary; and the projections become an obstacle insealing the forming die with a vacuum bag. Besides, there was alsoproposed, for example, a method for preventing the slipping of honeycombcore by fixing prepreg laminates using an adhesive tape, fixing pins, asand paper or the like. None of these methods, however, is practicalbecause of the problems such as increased number of steps, increasedcost for forming, or the like.

[0004] A method is also known which comprises filling the edge ofhoneycomb core with a filler curable at room temperature, and curing thefiller to increase the strength of the honeycomb core. In this method,however, widening of filling area for the prevention of crush invitesincreased panel weight, against the aim of lighter product weight.Moreover, because the cured filler is hard and fragile, the honeycombsandwich panel produced may suffer cracking when the panel has a curvedsurface.

[0005] Further, a method was proposed which comprises providing the edgeof a honeycomb core with a tapered portion to lower pressure in theforming process. The forming pressure lower than usual (about 6kgf/cm²), however, tends to invite insufficient forming owing topressure shortage in portions of skins where no honeycomb core isinterposed. Also, an increase in the degree of tapering at the edge ofhoneycomb core incurs an increase in oblique dead space in a taperedportion.

OBJECT OF THE INVENTION

[0006] Thus, the object of the present invention is to provide a methodfor producing a honeycomb sandwich panel suited for the production of apanel structure having a curved surface while preventing the crush of ahoneycomb core, thereby achieving simplification of the production stepsand reduction in product cost and weight.

DISCLOSURE OF THE INVENTION

[0007] As a result of intensive research in view of the above object,the inventors have found that a honeycomb sandwich panel constituted bya pair of prepreg laminate skins and a honeycomb core interposed betweenthe skins can be produced without crushing the honeycomb core by usingflat plate-shaped elastic jigs and flat plate-shaped press jigs duringpressurization, or by making wider a portion of the panel where a pairof prepreg laminate skins are overlapping each other. The presentinvention has been completed based on this finding.

[0008] The first method for producing a honeycomb sandwich panelaccording to the present invention comprises the steps of (a) mounting ahoneycomb core on a first prepreg laminate placed on a forming die, (b)superimposing thereon a second prepreg laminate to cover the honeycombcore, (c) mounting flat plate-shaped elastic jigs on a portion of theresulting assembly where the first prepreg laminate and the secondprepreg laminate are overlapping each other, (d) superimposing flatplate-shaped press jigs on the elastic jigs, and (e) heating theassembly under pressure for adhesion forming.

[0009] The second method for producing a honeycomb sandwich panelaccording to the present invention comprises the steps of (a) mounting ahoneycomb core on a first prepreg laminate placed on a forming die, (b)superimposing thereon a second prepreg laminate to cover the honeycombcore, and (c) heating the assembly under pressure for adhesion forming,wherein a portion of the assembly where the first prepreg laminate andthe second prepreg laminate are overlapping each other has a width of500 mm or more.

[0010] In the first method for producing a honeycomb sandwich panel, theelastic jigs are preferably made of a silicone rubber, and the pressjigs are preferably made of aluminum. Further, each of the elastic jigsand the press jigs preferably has a width of 40 to 250 mm and athickness of 0.3 to 2 mm. The forming die has a forming portion abovewhich the honeycomb core is mounted and an edge portion above which theelastic jigs and the press jigs are mounted, an angle between theforming portion and the edge portion being preferably 30 to 180°.

[0011] In the first and second methods for producing a honeycombsandwich panel, the adhesion forming is preferably conducted at apressure of 6 to 7 kgf/cm². The honeycomb core preferably has a taperedportion formed by chamfering, and the edge formed by the lower and uppersurfaces of the tapered portion, an angle between a lower surface and anupper surface of the tapered portion (angle of honeycomb edge) beingpreferably 30 to 45°.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a perspective view showing an example of the firstmethod for producing a honeycomb sandwich panel according to the presentinvention;

[0013]FIG. 2 is a perspective view showing an example of the firstmethod for producing a honeycomb sandwich panel according to the presentinvention;

[0014]FIG. 3(a) is a top view showing an example of the first method forproducing a honeycomb sandwich panel according to the present invention;

[0015]FIG. 3(b) is a fragmentary sectional view taken along the line A-Ain FIG. 3(a);

[0016]FIG. 4 is a fragmentary sectional view showing a manner in which ahoneycomb sandwich panel is produced on a forming die having a curvedsurface, using only a press jig without an elastic jig;

[0017]FIG. 5 is a fragmentary sectional view showing a case of using aforming die having a curved surface, in the first method for producing ahoneycomb sandwich panel according to the present invention;

[0018]FIG. 6 is a fragmentary sectional view showing a case of using aforming die having an angle θ₁ of 90° between the forming portion andthe edge portion, in the first method for producing a honeycomb sandwichpanel according to the present invention;

[0019]FIG. 7 is a sectional view showing a case of using a forming diehaving a curved surface, in the first method for producing a honeycombsandwich panel according to the present invention;

[0020]FIG. 8 is a fragmentary sectional view showing an example of thesecond method for producing a honeycomb sandwich panel according to thepresent invention; and

[0021]FIG. 9 is a sectional view showing an example of the combined useof the first and second production methods, using a forming die having acurved surface.

PREFERRED EMBODIMENTS OF THE INVENTION

[0022] The method for producing a honeycomb sandwich panel according tothe present invention will be explained hereinafter with reference toFIGS. 1 to 9. FIGS. 1 and 2 are perspective views each showing anexample of the first production method of the present invention. FIG.3(a) is a top view of FIG. 2, and FIG. 3(b) is a fragmentary sectionalview taken along the line A-A in FIG. 3(a).

[0023] In the first method of the present invention, a honeycomb core 3is mounted on a first prepreg laminate 2 (for an outer skin) placed on aforming die 1 as shown in FIG. 1. As shown in FIG. 2, a second prepreglaminate 2′ (for an inner skin) is then superimposed thereon to coverthe honeycomb core 3. On the portion of the resulting assembly where theprepreg laminates 2 and 2′ are overlapping each other are mounted flatplate-shaped elastic jigs 4 a, 4 b, 4 c and 4 d. With flat plate-shapedpress jigs 5 a, 5 b, 5 c and 5 d superimposed thereon, the assembly ofthe first prepreg laminate 2, the honeycomb core 3 and the secondprepreg laminate 2′ is heated under pressure for adhesion forming. Inthis method, the slipping of the prepreg laminates is prevented byfriction enhanced by the applied pressure and the edge effect of thepress jigs.

[0024] The shapes of the elastic jigs and the press jigs used in thefirst method of the present invention may be simple flat platesindependently of the shape of the forming die used. The crush of thehoneycomb core can be prevented by using the press jigs alone withoutelastic jigs. In using the press jigs alone, however, when the formingdie 1 is not flat plate-shaped and has, for example, a curved surface asshown in FIG. 4, insufficient forming takes place in the portion of theprepreg laminates where no honeycomb core is interposed, owing to (a)the presence of a space formed by the second prepreg laminate 2′ and thepress jigs and (b) the resulting pressure shortage. Even in such a case,the elastic jigs 4 as shown in FIG. 5 can transmit the applied pressureuniformly to a product to be formed, owing to the elasticity of the jigs4.

[0025] In FIG. 3(b), the width L₁ of the elastic jig 4 a and the widthL₂ of the press jig 5 a may each be 40 mm or more for sufficientprevention of crush and are preferably 40 to 250 mm for practicalapplication. The thickness of each elastic jig 4 a and each press jig 5a is preferably 0.3 to 2 mm, more preferably 0.5 to 1 mm, because thethickness of less than 0.3 mm gives no sufficient effects and thethickness of more than 2 mm makes it difficult to form the jig in apanel shape. Because the elastic jig 4 a and the press jig 5 a are verythin, they neither become obstacle in applying a vacuum bag nor breakthe vacuum bag.

[0026] The elastic jig 4 a is preferably made of an elastomer havingflexibility and heat resistance, more preferably of a silicone rubber.The press jig 5 a is preferably made of a material excellent in strengthand, rigidity and low in thermal expansion. As such a material, therecan be mentioned metals such as steel, stainless steel, aluminum and thelike; graphite; carbon fiber-reinforced resins, and so forth. The pressjig 5 a is most preferably made of aluminum.

[0027] The forming die used in the present invention has a formingportion above which the honeycomb core is mounted, and an edge portionabove which the elastic jig 4 a and the press jig 5 a are mounted. Anangle θ₁ between the forming portion and the edge portion is preferably30 to 180°. The angle θ₁ is more preferably 45 to 90°, because thehoneycomb core is sufficiently prevented from crushing even when thewidths L₁ and L₂ of the elastic jig 4 a and the press jigs 5 a are madesmaller. An example of the forming die having an angle θ₁ of 90° isshown in FIG. 6.

[0028] The first production method of the present invention using aforming die having a curved surface is exemplified in FIG. 7. Becausethe shape of the forming die used in the present invention is notcritical, the forming die may have, for example, a flat-plate shape or acurved surface. The forming die may be made of, for example, metals orcarbon fiber-reinforced resins.

[0029] The honeycomb core preferably has a tapered portion formed bychamfering. In FIG. 3(b), the edge angle θ₂ of the honeycomb core may be30 to 90°, and preferably 30 to 45° for sufficient prevention of thecrushing of the honeycomb core. In the method of the present invention,the edge angle of the honeycomb core can be made larger than those(usually 30° or less) in conventional methods. Therefore, the taperedportion (a dead space) can be made small, with the freedom of designenhanced.

[0030] The materials of the honeycomb core may be papers, papersimpregnated with resins, plastics, aluminum, steel, etc. Aramidfiber-reinforced thermosetting resins having high specific strength arepreferably used. In that case, the thermosetting resins are preferablyphenolic resins.

[0031] As the prepreg, there can be used those obtained by impregnatinga woven fabric of reinforcing fibers such as carbon fibers, glassfibers, aramid fibers or the like with a thermosetting resin or athermoplastic resin. The thermosetting resin is preferably an epoxyresin or the like, and the thermoplastic resin is preferably nylon orthe like.

[0032] By the method of the present invention, the crush of honeycombcore edge can be prevented even under high-pressure forming. The formingpressure employed in the present invention is preferably 5 to 7 kgf/cm²,more preferably 6 to 7 kgf/cm². Such forming at a pressure higher thanin conventional methods (using 3 to 4 kgf/cm²) incurs no insufficientforming due to pressure shortage.

[0033] The heating temperature is preferably 120 to 250° C. in general,although it may slightly differ depending upon the kind of thethermosetting or thermoplastic resins contained in the prepreg.

[0034] In the second production method of the present invention, neitherelastic jigs nor press jigs are used unlike the first production method,and instead the width of the panel portion where the lower prepreglaminate skin and the upper prepreg laminate skin are overlapping eachother is made larger, thereby intensifying the friction of the prepregsand thus preventing the slipping of the prepregs in the laminates,leading to the prevention of the crush of the honeycomb core.

[0035]FIG. 8 is a fragmentary sectional view showing an example of thesecond production method of the present invention. First, a honeycombcore 3 is mounted on a prepreg laminate 2 as a lower skin placed on aforming die 1, and a prepreg laminate 2′ is superimposed as an upperskin thereon to cover the honeycomb core 3. The resulting assembly isheated under pressure to cause adhesion forming. In order to effectivelyprevent the crush of the honeycomb core, the width L₃ of the portionwhere the prepreg laminate 2 and the prepreg laminate 2′ are overlappingeach other, is preferably 500 mm or more, more preferably 500 to 600 mm.Further in the present method, cost reduction is achieved because nojigs or the like are used.

[0036] All of the forming die, the honeycomb core, the prepreglaminates, the forming pressure, the forming temperature, etc. used inthe second production method may be the same as those used in the firstproduction method in shape and material.

[0037]FIG. 9 is a sectional view showing an example of the forming of ahoneycomb sandwich panel using a forming die having a curved surface. Anelastic jig 4 and a press jig 5 are mounted at the left side of thehoneycomb core 3, because the width of the panel portion, where aprepreg laminate 2 (a lower skin) and a prepreg laminate 2′ (an upperskin) are overlapping each other, is small at the left side of thehoneycomb core 3, while neither elastic jig 4 nor press jig 5 is used atthe right side of the honeycomb core 3 because the width of the aboveoverlapping portion is sufficiently large at the right side of thehoneycomb core 3. As in FIG. 9, the first and second production methodsof the present invention may be used in combination.

[0038] In the production method of the present invention, an adhesivemay be used if necessary between the individual layers of the panel; anda vacuum bag, a sealing agent or the like may be used during adhesionforming.

[0039] Though the method for producing a honeycomb sandwich panelaccording to the present invention has been explained above withreference to the attached drawings, the present invention is notrestricted thereto, and any modifications are possible unless the scopeand spirit of the present invention are changed.

[0040] As described in detail above, the method for producing ahoneycomb sandwich panel according to the present invention can preventthe crush of the honeycomb core during forming by using elastic jigs andpress jigs all of simple shape, or by making large the width of thepanel portion where the prepreg laminates are overlapping each other.Further in the present production method, the production steps can besimplified with product cost and weight reduced, because it is notnecessary to process a forming die into a complex shape or to use afixing jig, a filler or the like. Furthermore in the present productionmethod, insufficient forming due to pressure shortage can be avoidedbecause forming at an ordinary pressure (about 6 kgf/cm²) is possible.The method of the present invention is also applicable to the productionof a honeycomb sandwich panel having a curved surface.

What is claimed is:
 1. A method for producing a honeycomb sandwichpanel, comprising the steps of (a) mounting a honeycomb core on a firstprepreg laminate placed on a forming die, (b) superimposing thereon asecond prepreg laminate to cover said honeycomb core, (c) mounting flatplate-shaped elastic jigs on a portion of the resulting assembly wheresaid first prepreg laminate and said second prepreg laminate areoverlapping each other, (d) superimposing flat plate-shaped press jigson said elastic jigs, and (e) heating said assembly under pressure foradhesion forming.
 2. The method for producing a honeycomb sandwich panelaccording to claim 1, wherein said elastic jigs are made of a siliconerubber, and said press jigs are made of aluminum.
 3. The method forproducing a honeycomb sandwich panel according to claim 1, wherein eachof said elastic jigs and said press jigs has a width of 40 to 250 mm anda thickness of 0.3 to 2 mm.
 4. The method for producing a honeycombsandwich panel according to claim 1, wherein said forming die has aforming portion above which said honeycomb core is mounted and an edgeportion above which said elastic jigs and the press jigs are mounted,and wherein an angle between said forming portion and said edge portionis 30 to 180°.
 5. The method for producing a honeycomb sandwich panelaccording to claim 1, wherein a pressure of 6 to 7 kgf/cm² is appliedfor adhesion forming.
 6. The method for producing a honeycomb sandwichpanel according to claim 1, wherein said honeycomb core has a taperedportion formed by chamfering, an angle between the upper and lowersurfaces of said tapered portion (angle of said honeycomb edge) being 30to 45°.
 7. A method for producing a honeycomb sandwich panel, comprisingthe steps of (a) mounting a honeycomb core on a first prepreg laminateplaced on a forming die, (b) superimposing thereon a second prepreglaminate to cover said honeycomb core, and (c) heating the resultantassembly under pressure for adhesion forming, wherein a portion of saidassembly where said first prepreg laminate and said second prepreglaminate are overlapping each other has a width of 500 mm or more. 8.The method for producing a honeycomb sandwich panel according to claim7, wherein a pressure of 6 to 7 kgf/cm² is applied for adhesion forming.9. The method for producing a honeycomb sandwich panel according toclaim 7, wherein said honeycomb core has a tapered portion formed bychamfering, an angle between the upper and lower surfaces of saidtapered portion (angle of said honeycomb edge) being 30 to 45°.